1. Field of the Invention
The present invention relates to a lithography technique for forming a micropattern required for an LSI or VLSI and, more particularly, to a mask defect repair system and method for repairing a defect generated on a mask subjected to pattern exposure.
2. Description of the Related Art
A focused ion beam (to be abbreviated as an FIB hereinafter) technique is used as a tool for observing or forming a micropattern because this technique can focus a beam on the order of submicrons.
When the FIB technique is used as a tool for observing a micropattern, imaging of the observation surface is performed with an FIB. This method uses a fact that the emission efficiency of secondary particles which are emitted upon FIB irradiation changes depending on the material and shape of a sample. More specifically, a deflector is used to scan an FIB on the sample surface, and the secondary particle intensity is detected at each scanning point. An image is formed on the basis of the secondary particle intensity obtained at each point.
In this case, for the purpose of removing noise, a simple noise removing method is used. That is, a low-pass filter is applied, or when a scanning operation is to be repeatedly performed, the secondary particle intensity obtained first at each point is integrated with that obtained upon subsequent beam scanning, thereby forming a sharper image.
On the other hand, as a tool for forming a micropattern, an FIB is used to repair a circuit for debugging in LSI development, repair a defect generated on an exposure mask, form a quantum wire, or prepare a transmission electron microscope sample.
Since an FIB is a high-energy beam, the processed sample may be damaged. In etching using an FIB, the selectivity between materials to be etched can hardly be set.
To repair a bump defect (a defect in which an unnecessary film remains) generated on an exposure mask, a method must be developed such that a beam is irradiated on only a defect region conforming to the shape of the bump defect generated on the mask, and the dose is controlled in correspondence with the film thickness of the defect because the shape and thickness of the defect are nonuniform.
As a method of irradiating a beam on only the defect region conforming to the shape of the defect, an adaptive beam blanking method is known, as disclosed in a reference (T. D. Cambria et al. "Mask and Circuit Repair with Focused-Ion Beams", Solid State Technology, p. 113 (1987)). In this method, a particle beam is irradiated on a mask before processing, and an image formed by secondary particles emitted at this time is fetched. A beam is irradiated on only a region where the secondary particle intensity is at a predetermined level or more or at a predetermined level or less, thereby removing the unnecessary substance.
In addition, a method of controlling the beam dose in accordance with the thickness of a defect is disclosed in Jpn. Pat. Appln, KOKAI Publication No. 63-141060, in which a sample surface is divided into small regions, and a beam is irradiated in units of small regions.
As an exposure mask, a mask prepared by forming an opaque film consisting of Cr or MoSi on a quartz substrate having a transmissivity with respect to exposure light is conventionally used. Along with an advance in micropatterning of LSI patterns in recent years, a development associated with a stepper has been made in, e.g., shortening the wavelength of exposure light, increasing an NA (Numerical Aperture), or realizing a modified illumination system. In addition, a phase shifting mask has been developed.
An alternated type phase shifting mask has a conventional structure in which an opaque film consisting of Cr or MoSi is formed on a quartz substrate. In addition to this structure, a phase shifter is formed to invert the phase of an adjacent opening portion. With this structure, light beams transmitted through adjacent openings interfere with each other, thereby achieving pattern separation.
In a halftone mask, a halftone film is formed on a substrate. A light beam transmitted through the substrate interferes with a light beam transmitted through the halftone film such that these light beams have opposite phases, thereby achieving pattern separation. As the material of the halftone film, SiN.sub.x, MoSiO, MoSiON, WSiO, C, Cr, CrO.sub.x, Si, or the like is used.
However, when a defect on a phase shifting mask is to be repaired using the FIB technique, the following problems are posed. When an FIB is used as a tool for processing a micropattern, only a simple noise removing method is used, as described above, so that noise is included in the image. When the adaptive beam blanking method is to be used to repair a bump defect generated on a mask which has a low contrast between a region where a beam is to be irradiated and a region where the beam is to be blanked, a region which must be irradiated with a beam may not be irradiated and vice versa.
Particularly, as for a halftone mask corresponding to deep UV light (wavelength: 248 nm), the halftone film for obtaining a desired transmissivity and refractive index is formed of an intermediate material between a metal and an insulating material, as disclosed in a reference (S. Itoh at al. "Optimization of optical properties for single-layer halftone masks", SPIE 2197, 99 (1994)), so that the halftone film corresponding to exposure light with a short wavelength is close to an insulating material. For this reason, when imaging by secondary particles is performed for a halftone film corresponding to exposure light having a short wavelength, a contrast difference from the quartz substrate cannot be attained. Therefore, the influence of noise becomes large, resulting in a difficulty in recognizing the material of the defect.
To reduce noise, the number of times of imaging operations is increased, or a dose for one scanning operation is increased to increase the total dose. However, in a repair using an FIB, the sample is damaged by beam irradiation. Consequently, the transmissivity is decreased, and sufficient light intensity cannot be obtained, so a normal pattern cannot be formed. Even for samples damaged to the same degree, the transmissivity is decreased as the wavelength is shortened. To cope with an advance in micropatterning, the wavelength of a light source tends to become shorter, so this problem becomes more serious.
A problem that various kinds of noise are included in the secondary particle intensity occurs not only during imaging before processing but also during etching end point detection. Particularly, when gas assisted etching is performed to avoid damage caused by beam irradiation, beam irradiation not only activates etching but also eliminates an adsorbed gas. Therefore, to increase the etching rate, the beam irradiation time per pixel must be much shorter than that in imaging. For this reason, the S/N ratio of resultant secondary particles further decreases in etching.
The shape of an FIB is not an ideal rectangle and has an extending intensity distribution (Gaussian distribution). For this reason, a region actually processed does not simply correspond to a beam irradiation position, and the substrate outside the unnecessary portion is undesirably etched. Particularly, in gas assisted etching using an etching gas, a portion outside an actual beam irradiation region is also etched depending on the selected beam condition. In addition, an image corresponding to the edge portion of a film formed on a mask has a width. Therefore, the unnecessary mask material is undesirably left depending on the method of setting a particle beam irradiation region.
Conventionally, when a defect generated on a mask with a low contrast is to be repaired using an FIB, the defect region cannot be properly recognized because the image of the mask formed by beam scanning includes noise. Particularly, when imaging is performed for a halftone mask corresponding to exposure light with a short wavelength by secondary particles, the contrast difference from the quartz substrate cannot be obtained. Consequently, the influence of noise becomes large, resulting in a difficulty in recognizing the material of the defect.
When the number of times of imaging operations is increased or a dose for one scanning operation is increased to reduce noise, the transmissivity of the substrate is decreased, and no sufficient light intensity can be attained, so a normal pattern cannot be formed.